fastbootd: Allow flashing the cache partition.

This fixes the following denial:
  avc:  denied  { write } for  pid=332 comm="fastbootd" name="mmcblk0p35" dev="tmpfs" ino=11234 scontext=u:r:fastbootd:s0 tcontext=u:object_r:cache_block_device:s0 tclass=blk_file permissive=0
  avc:  denied  { ioctl } for  pid=294 comm="fastbootd" path="/dev/block/mmcblk0p35" dev="tmpfs" ino=6953 ioctlcmd=0x1277 scontext=u:r:fastbootd:s0 tcontext=u:object_r:cache_block_device:s0 tclass=blk_file permissive=0

Bug: 150112538
Test: fastboot flash cache on non-A/B device
Test: fastboot erase cache on non-A/B device
Change-Id: Ib2288b42f2bb47e83b1476319669d6c2719db2ec
Merged-In: Ib2288b42f2bb47e83b1476319669d6c2719db2ec
diff --git a/public/fastbootd.te b/public/fastbootd.te
index a0152d4..f10e649 100644
--- a/public/fastbootd.te
+++ b/public/fastbootd.te
@@ -45,6 +45,7 @@
   allow fastbootd dm_device:chr_file rw_file_perms;
   allow fastbootd dm_device:blk_file rw_file_perms;
 
+  allow fastbootd cache_block_device:blk_file rw_file_perms;
   allow fastbootd super_block_device_type:blk_file rw_file_perms;
   allow fastbootd {
     boot_block_device
@@ -67,6 +68,7 @@
     metadata_block_device
     userdata_block_device
     dm_device
+    cache_block_device
   }:blk_file ioctl { BLKSECDISCARD BLKDISCARD };
 
   allow fastbootd misc_block_device:blk_file rw_file_perms;